National achievement data show that elementary school students in the United States, particularly those from low socioeconomic backgrounds, have weak math skills (National Center for Education Statistics 2009). In fact, data show that, even before they enter elementary school, children from disadvantaged backgrounds are behind their more advantaged peers in basic competencies such as number-line ordering and magnitude comparison (Rathburn and West 2004). Furthermore, after a year of kindergarten, disadvantaged students still have less extensive knowledge of mathematics than their more affluent peers (Denton and West 2002). This study examines whether some early elementary school math curricula are more effective than others at improving student math achievement in disadvantaged schools. A small number of curricula, which are based on different approaches for developing student math skills, dominate elementary math instruction--7 curricula make up 91 percent of those used by K-2 educators, according to a 2008 survey (Resnick et al. 2010). Little rigorous evidence exists to support one approach over another, however, which means that research does not provide educators with much useful information when choosing a math curriculum to use. The key findings in this report include the following: (1) Teachers used their assigned curriculum, and the instructional approaches of the four curriculum groups differed as expected; (2) Math instruction varied in other notable ways across the curriculum groups; (3) In terms of student math achievement, the curriculum used by the study schools mattered; and (4) The curriculum used in different contexts also mattered, and some of these findings are consistent with findings based on all students whereas others are not. (Contains 1 figure and 6 endnotes.) [For the full report, see ED512551.]